This invention relates generally to relay valves and deals more particularly with a manually set relay valve of the type used in fluid control systems such as safety systems for oil and gas wells.
Pilot operated relay valves are commonly used in safety systems which shut-in well production lines or other flowlines in the event of unduly high or low pressure conditions. The relay is operated by pilot pressure and includes a slide valve which controls fluid flow to and from a gate valve actuator. The gate valve is closed by the actuator if the flowline pressure, as sensed by a pair of pressure sensitive pilots, is outside of the operating range of the safety system. Once the relay has moved to the closed position due to cut off of the pilot pressure, it is reset manually after correction of the problem which caused the unusual pressure condition in the flowline.
Although most relay valves have been pull type valves in which the slide valve is pulled outwardly to its set operating position, push type relays have also been proposed, as exemplified by U.S. Pat. Nos. 4,004,610 and 4,145,025. Because the push type relay has a slide valve which is pushed inwardly when it is set in the normal operating position, the slide valve is almost fully retracted within the valve body and only a small part is exposed under normal operating conditions. The push type slide valve is thus effectively shielded from foreign materials which could otherwise form deposits that might interfere with proper operation of the relay, particularly when it is used in an offshore facility or another location where it is exposed to salt, sand, or other foreign substances.
For safety reasons, it is desirable for the relay valve to be constructed such that it remains in the fired or closed position until it is manually reset. Thus, a resumption of pilot pressure following cut-off does not open the relay, and the problem which caused the relay to fire can be corrected before the flowline is again open to flow. The arrangements that have been used in the past to achieve such a lockout of the relay have been less than satisfactory in a number of respects. For example, the relay shown in U.S. Pat. No. 3,877,484 requires an unduly complex valve body and slide valve, while the arrangements of U.S. Pat. Nos. 4,094,340 and 4,145,025 suffer from the same problems as well as relying on close tolerances and the integrity of O-ring seals to establish a necessary pressure condition in the valve.
It has also been proposed to provide relay valves with visual indicators, as disclosed in U.S. Pat. Nos. 4,137,942 and 4,121,615. However, only pull type relays are disclosed in these patents, and push type relay valves such as those shown in U.S. Pat. Nos. 4,004,610 and 4,145,025 are not equipped with visual indicators. The indicators which have been proposed for relay valves have not included a positive means for limiting the movement of the indicator mechanism. Therefore, high fluid pressure applied to the indicator can damage the indicator linkage by moving it beyond its intended operating range.
Relay valves most often have one port for receiving pilot fluid to hold the slide valve open and another separate supply or inlet port for receiving actuator fluid which is directed to the valve actuator in order to hold the gate valve open under normal conditions. The pilot fluid and actuator fluid are at different pressures in most situations and are thus isolated from one another. Another type of relay has a single inlet port and uses the same fluid to maintain the slide valve open and to supply the actuator. An example of this latter type of relay can be found in U.S. Pat. No. 4,004,610.